How to

24 March 2021

A Guide to Defoliation

Growers searching for a technique to put their plants in the best position to produce higher yields should consider cannabis defoliation. This delicate procedure is the strategic removal of leaves from the plant that can dramatically impact health and production for better or worse, depending on how and when it is done. 

Read on to find out the principles behind defoliation, how to assess a plant for defoliation and the best way to defoliate.

Why defoliation works: increased light exposure and improved plant health

Growers increase the plant’s overall light exposure through defoliation. Bigger yields are produced as more bud sites receive more light penetration and the opportunity to develop into full flowers rather than small “popcorn” nugs found below the canopy. Plants react very positively to methods that increase the amount of light exposure, which will encourage the plant’s overall photochemical production, resulting in higher cannabinoid concentration, fuller trichomes, and “loud” terpene profiles.

By defoliating, the grower also increases aeration below the canopy. The risk of mould, mildew, pests, and pathogens is reduced by creating more space for airflow. Removing larger leaves also reduces overall humidity and redirects energy towards flowering.

Cultivators can develop a better eye for their plants’ health and characteristics by spending time defoliating their plants. This can lead to many positive outcomes, such as timely identification of pests, viruses, and diseases and identifying desired traits. The grower who removes extra leaves that do not directly support bud development will help their plants make more efficient use of limited resources and light.

Less is more: the role of leaves in plant growth and defoliated plants

Cannabis plants need their famous leaves for photosynthesis to occur as the crucial light-absorbing chemical chlorophyll is found most abundantly in leaves. As such, the challenge of defoliating is for the grower to remove fewer leaves than they think they will need because removing too many leaves will stunt the plant’s growth. Plants with a greater surface area of leaves to absorb light will grow more vigorously.

Conversely, bud sites found lower down the stalks of cannabis plants will not develop to the same level as their canopy counterparts due to being shaded by leaves. Growers who strategically defoliate will expose bud sites to direct light and airflow, which will increase their yield and bud density. At the same time, defoliation will cause the plant to redirect its resources away from superfluous leaves and trigger surges in plant growth.

Why should growers remove leaves? After all, they are a source of photosynthetic production and nutrient storage, which can be tremendously valuable, especially for plants outdoors that need to contend with environmental stressors such as pests and inconsistent nutrient availability during the growing season.

In a controlled grow room environment, these challenges are mitigated. If the grower is doing their job correctly, the plants will never want nutrients, the conditions are stable and monitored, and proper precautions are taken to minimise the risk of pest infestations. Thus, the need for leaves isn’t as pronounced indoors, and growers who defoliate strategically optimize light availability in the grow space and redirect energy towards developing bud sites.

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Assessing plants for defoliation

Defoliating healthy leaves stresses cannabis plants as light absorption and nutrient availability are suddenly reduced. The grower can ensure a strong response by defoliating their healthiest plants. Assessing plant health over their life cycle will help determine whether or not the plants would react well to defoliation.

Have the plants been consistently vigorous during their growth? If assessing multiple plants, the grower should take note of any that have stunted or have grown slower than their counterparts. Plants that have been slow to develop or bounce back from stressors are not ideal candidates for defoliation.

Are there any plants that have shown signs of nutrient deficiency, appear fragile, or have shown to be prone to pests and disease during veg? These plants are already stressed and should not be defoliated. Instead, the grower should focus on supplementing their nutrients or implementing a robust pest management strategy and then defoliating during flower after the plants have stabilised.

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If plants are healthy and vigorously growing with upright stems in a well-draining planting medium, with dense green leaves and several nodes, then they are prime for defoliation. Another easy way for the grower to tell if a healthy plant is ready is by examining the density of leaves underneath the canopy: are leaves laying on top of one another on the inside of the plant, blocking light from getting through to nodes on the main stem or branches? These leaves should be removed.

Similarly, if during plant inspection the grower finds mould on the leaves on the inside of the plant underneath the canopy, then these leaves must be removed to prevent disease from spreading to the rest of the plant and to create airflow. The grower can employ a quick eye-test to assess where to start defoliating. If plants are so bushy and leafy that entire portions of the plant are covered in shadow by dense leaves or leaves are so thick that the grower can’t see through to the other side of the plant, then it’s time to defoliate and create direct channels for light to reach shaded nodes.

Defoliation is not recommended for the novice grower. Decisions about assessing whether or not plants can be defoliated will come easier to the grower who has developed their eye for the growth habits and quirks of their plants. If the novice grower is committed to defoliating, they should only defoliate during veg and not during the flowering stage.

Grower defoliating cannabis plant with secateurs

How to Defoliate – Leaves Only, and Sparingly

Indoor growers trying to make the most of defoliating their plants should do so during the vegetative stage, one week before flipping them to flower. It is recommended to remove far fewer leaves than one would think – defoliating 10-20% of a single plant’s total foliage is a good rule of thumb. The grower can break defoliation down to three categories of leaves, based on size, location and colour:

1 . The most obvious targets are big fan leaves. These can be found in the canopy of the plant closer to the main stem and branches, shading everything underneath them. Removing two or three can dramatically increase light exposure to bud sites lower down on the plant, giving them the chance they need to develop into large, desirable nugs. Avoid removing the uppermost fan leaves in the canopy as these are necessary to develop top colas

2 . Any leaves that are growing inwards, towards the stem and branches rather than outwards towards the lights, are also good candidates for removal. A helpful way to visualise is to imagine a dome beginning at the top of the canopy and stretching down to the middle of the plant. Any leaves on the edges of the dome should be left alone as they are already exposed to light, while leaves on the inside of the dome close to stems and branches can be defoliated.

3 . Yellowing or dead leaves, regardless of the location. These will waste the plant’s energy that could be put to better use elsewhere for development. If left on, they can be a vector for pests and disease.

If there is any doubt about whether to remove a leaf, the best outcome is to leave it where it is, as leaves can be removed but not put back. After the veg-phase defoliation, let the plants recover for three to five days before flipping them into flower.

After the third or fourth week of flowering, a final defoliation session can increase yields by firming up previously shaded bud sites. The grower should start low on the plant so as not to deprive top colas of photosynthetic capacity and should take care when handling plants so as not to agitate their developing flowers too much. A low, wide canopy that maximises the penetration of grow lights to more nodes is essential to bountiful indoor harvests.

Low, wide indica cultivars such as those found in Marijuana Grow Shop’s impressive collection are ideal candidates for increasing yield via defoliation. These robust phenotypes are sourced from dedicated commercial growers and the world’s leading seed banks. Whether you’re a first-time grower or have many harvests under your belt, head over to MGS, explore their diverse collection of feminised, autoflowering, and exclusive regular seeds, and start your next project the right way.

Post author
Martin
Martin is a production horticulturist with experience in commercial cannabis cultivation and sustainable farming from his time with Emerald Cup Award-winning farmers Esensia Gardens in northern California's Emerald Triangle.
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